the rule of Erling Skakke and King Magnus. Some materialists among modern historians have tried to read a form of class struggle into Sverre and the Birkebeiner's fight against Erling and Magnus. However, the extent to which Sverre's men actually represented the impoverished strata of the population remains disputed. It is clear that most of the lendmenn—the nobility of the time—sided with King Magnus, but Sverre also quickly won several of them over to his side. In any event, the Birkebeiner did not try to change the social order of society; they merely wanted to place themselves at its top. In 1179 Sverre won an important victory in the battle at Kalvskinnet (Slaget ved Kalvskinnet) on the outskirts of Nidaros, where Erling Skakke was killed. From that point, the Trøndelag region with Nidaros at its center became a stronghold of Sverre. King Magnus continued the fight after the death of his father and refused several offers from Sverre to divide the kingdom between them. Sverris saga, which was written by Sverre's supporters, makes much of how popular Magnus was among the common people and how this made Sverre's fight against him all the more difficult. The war between Sverre and Magnus raged on for several years, and Magnus at one point had to seek refuge in Denmark. The Battle of Fimreite, a final naval battle during 1184 in the Sognefjord, resulted in the death of King Magnus and victory for King Sverre. Sverre was to rule Norway until 1202, but was unable to achieve long periods of peace. The Church, allied to King Magnus and Erling Skakke, remained virulent in its opposition to Sverre throughout his reign. In 1190 the archbishop, Eirik Ivarsson, fled the country and in 1194 he received papal support to excommunicate Sverre and order the country's remaining bishops to join him in exile in Denmark, which they did. By then Sverre had been able to coerce one of his strongest opponents, Bishop Nikolas Arnesson of Oslo, to crown him in Bergen in 1194. In 1198 Pope Innocent III placed Norway under interdict. Although Sverre forged letters to show that his excommunication had been lifted, he in fact remained excommunicated until his death. Several pretenders arose to challenge Sverre. Among the most serious was Jon Kuvlung, a purported son of King Inge Crouchback. He was named king in 1185 and killed in battle in Bergen three years